1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by an electrophotographic method and to a method of controlling this apparatus.
2. Description of the Related Art
Digital multifunction peripherals that can be utilized as a copier, printer and facsimile machine have become widespread. In the case of low-end products of this type in particular, there is a tendency to assign greater importance to the conserving of space. In order to achieve space savings, use is made of a oblique incidence method in which laser beam diagonally enters a reflecting surface of a polygonal mirror obliquely.
FIG. 4 is a diagram useful in describing the oblique incidence method.
As shown in FIG. 4, laser beam emitted from a laser emitting device 400 is reflected by a polygonal mirror 401 and is scanned across a photosensitive drum 402 to thereby form an electrostatic latent image on the surface of the photosensitive drum 402. The scanning surface of the polygonal mirror 401 for scanning the laser beam is placed at a height different from that of the laser emitting device 400 so that the laser beam diagonally enters a reflecting surface of a polygonal mirror obliquely.
In this method, the distances a, b from the center of the polygonal mirror to the center and edge, respectively, of the scanning surface that reflects the laser beam are different from each other. The difference between these distances appears as an offset in the height direction at the surface of the photosensitive drum 402.
FIG. 3 is a diagram useful in describing a displacement in laser exposure position on a photosensitive drum ascribable to a difference in position on the scanning surface of the polygonal mirror that reflects the laser beam in the above-described oblique incidence method.
A dotted line 300 in FIG. 3 indicates the ideal scanning path of the laser beam. A solid line 301, on the other hand, indicates a scanning path ascribable to the influence of the above-mentioned variation along the height direction.
In order to cancel out this variation in the laser path along the height direction in the oblique incidence method, a correcting lens is placed on the scanning path of the laser beam, thereby canceling out the amount of fluctuation to realize a high image quality. However, machining of the correcting lens, assuring the precision of the lens and the fact that an adjustment process for maintaining the optical system in the desired state takes time all have an influence upon manufacturing cost. Further, a variation along the height direction can occur not only in the oblique incidence method but even in a system in which the laser beam enters upon the axis of rotation of the polygonal mirror perpendicularly.
FIGS. 7A and 7B are diagrams describing such occurrence of a variation in the laser path along the height direction at the surface of a photosensitive drum. It should be noted that portions in FIGS. 7A, 7B similar to those shown in FIG. 4 are designated by like reference characters.
FIG. 7A illustrates an ideal optical system. Here the photosensitive surface of the photosensitive drum 402 is irradiated at a constant height irrespective of distance from the axis of rotation of the polygonal mirror 401 to the surface thereof.
FIG. 7B, which illustrates an extreme case in order to facilitate understanding, shows how the axis of rotation of the polygonal mirror 401 incorporates a mounting error. In this case, in a manner similar to that of the oblique incidence method, a variation in height occurs at the surface of the photosensitive drum 402 owing to the scanning position of the laser beam. This affects image quality. Such error is a problem even in high-end models.
A digital correcting method has been proposed as a technique for dealing with curvature of laser scanning on the surface of a photosensitive drum.
FIG. 5 is a diagram useful in describing this correcting method.
The dotted line indicates an ideal scanning path on a photosensitive drum. Solid lines 500 to 502 indicate actual scanning paths that pass within ±0.5 line of the ideal scanning path indicated by the dotted line. In the vicinity of both ends of the scanning lines, the solid line 500 comes closest to the ideal scanning path. In the vicinity of the center, on the other hand, the solid line 502 comes closest to the ideal scanning path. Accordingly, the area scanned by the laser beam is divided into areas A, B and C, the laser beam is changed over in the following manner: solid line 500 (area A)→solid line 501 (area B)→solid line 502 (area C)→solid line 501 (area B)→solid line 500 (area A), whereby the position of the scanning line formed on the photosensitive drum is made to approach the ideal scanning line. This method is described in Japanese Patent Application Laid-Open Nos. 02-050176, 2005-304011 and 2003-276235.
In this case, the position of each solid line and the ideal scanning path do not coincide at all. The pixel data that is output at each scanning position therefore is not output as is as line data but is output as interpolation data obtained by interpolation between adjacent-line data. Such shifting of line data by interpolation in the sub-scanning direction is a technique having a high level of difficulty. In a low-cost product in particular, uniformity of the thickness of fine lines tends to be lost in view of the poor tone reproduction of very small dots.
FIGS. 6A to 6D are diagrams describing this situation.
In FIGS. 6A to 6D, each square indicates one dot formed by PWM (Pulse-Width Modulation). FIG. 6A illustrates an ideal fine line having a width of one dot. FIG. 6B shows an example of changeover of line data and a correction in which interpolation processing is not carried out. In the case of FIG. 6B, the fine line is one in which jaggies appear conspicuously at portion where the line data is changed over. FIG. 6C illustrates a case scanning curvature has been corrected by interpolation. Here the portions where the line data has been changed over by interpolation of the central portion become thick, and therefore it is difficult to reproduce thickness uniformly. For this reason, a technique has been proposed in which the range of a correction by interpolation at a portion where line data has been changed over is made the smallest possible interval, as illustrated in FIG. 6D, thereby obtaining an image in which the thickness of a fine line is made uniform without producing jaggies. In FIGS. 6A to 6D, the width of each square corresponds to the pulse width of a PWM signal.
When a color image is formed, generally images of a plurality of colors such as cyan, magenta, yellow and black are superimposed. However, the scanning paths of the beam that forms each of the images of the plurality of colors are different from one another. This means that in a case where processing is executed for outputting image data for forming dots of each color on the ideal scanning line, the changeover point of the line data is different for every item of data of each color. Owing to the fact that the changeover point of the line data thus differs for each item of image data of the plurality of colors, a color shift and color unevenness become conspicuous in the vicinity of the changeover points of the line data.
An increase in the speed of image formation in recent years has been accompanied by use of a laser system in which exposure is performed by arraying a plurality of lasers in the sub-scanning direction. In this case also where use is made of such lasers, it is required that the optical-path lengths from the laser emitting source to the surface of the photosensitive member along the main scanning direction be equalized and, at the same time, it is required that the scan length between the lasers arrayed in the sub-scanning direction and the photosensitive member also be equalized. Correction of the scan length depends upon an optical correction, which uses an f-θ lens, and mechanical precision. However, since such a correction requires a high degree of precision in terms of lens manufacture, it is a cause of higher cost.
Further, owing to improvements in the resolution of image formation, a difference in the optical-path lengths of multiple lasers that was allowed in the past has now come to have an effect upon the output image and can no longer be ignored. Further, in an image forming apparatus having a plurality of photosensitive members, a structure for adjusting scanning magnification on each photosensitive member is quite complex and adjustments are necessary. This leads to an increase in cost. Accordingly, a technique for revising an inappropriate scanning position on the photosensitive member by manipulating the laser beam lighting pattern has been proposed (see Japanese Patent Application Laid-Open No. 2005-96351).
However, if a revision of the scanning position on a photosensitive member is performed in an interval where a correction based upon pixel interpolation in the sub-scanning direction is applied in the vicinity of the changeover point of line data, the thickness of fine lines becomes uneven and the density of the image formed becomes non-uniform. This leads to a decline in image quality.
In image formation based upon image data, it is necessary to execute image scaling processing, namely adding on pixel data or deleting pixels in accordance with the size of the image formed and the volume (resolution) of image data. In a case where an image is formed using the above-described image forming apparatus based upon image data that has been subjected to such scaling processing, processing that changes over the line data for correcting the deviation between the actual scanning position of the laser beam and the ideal scanning position, in the manner described above, is similarly required. It should be noted that if pixel data added on anew or deleted pixel data exists at the line-data changeover position, then interpolation processing at the portion where the line data changes over is executed. However, pixel data added on or pixel data deleted is pixel data that did not exist in the original image data. If processing that is based upon pixel data of a position that did not originally exist or from which a pixel was removed is executed, then this will invite a decline in image quality.